How to send ether from a user to another?

Question

This should be really easy but I can't seem to find the way to do it, thanks to the great documentation. I'm thinking I'm missing something big.

I have a function that buys ownership of something. I want to remove amount ether to msg.sender (user a) and give them to owner (user b) (minus fees).

Can I do it directly? Do I have to first let a send ether to the contract and then send b the same amount, or is there a function to transfer directly?

I assume owner.transfer(amount) (or .send(amount)) sends ether from the contract to the owner, but then how do I get ether from a?

I'm using Solidity by Example as a model, but in the Blind Auction example I can't seem to find what function actually sends ether from the bidder to the contract.

Answer 1

You can't just take somebody's ether, so yes, they need to send it to the contract first, but this can all be done in a single contract call. E.g.

// "payable" means ether can be sent to this function
function buyIt() public payable {
    // msg.value is how much ether was sent
    require(msg.value == price);

    // send the ether to "owner"
    owner.transfer(msg.value);

    // msg.sender is the new "owner"
    owner = msg.sender;
}

When calling this function, the caller must attach price ether. This means setting the "value" field of the transaction.

I think this blog post will help: https://programtheblockchain.com/posts/2017/12/15/writing-a-contract-that-handles-ether/.

EDIT

You linked to a very old version of the Solidity documentation, before payable was required to allow ether to be sent. The current version shows a payable bid function. That's where ether is transferred to the contract.

Answer 2

How about web3 sendTransaction method? it has "to" option.

https://github.com/ethereum/wiki/wiki/JavaScript-API#web3ethsendtransaction